The present invention relates to an automatic choke system of an internal combustion engine for an motor vehicle wherein a choke valve is automatically opened by current supplied to a heater, and more particularly to a system wherein the opening degree of the choke valve is controlled at cold engine operation.
The automatic choke system in general comprises a thermostatic spring, a heater made of electric heating material such a nichrome, a positive temperature coefficient (PTC) element, and ceramics, for heating the thermostatic spring. With regulations on emission control in recent years, the choke valve is set taking emission control into consideration. In order to meet a requirement of emission control, the valve is designed to open early so as to prevent extreme enrichment of the air-fuel mixture for the engine.
The choke valve is controlled to close at all times except at coolant temperatures higher than a predetermined temperature (for example, 80.degree. C.). Accordingly it is desirable that the system have characteristics of controlling the choke valve at low coolant temperature, for example about 0.degree. C., as well as at normal temperature (about 25.degree. C.). However, the air-fuel ratio is liable to be increased to supply a lean mixture at lower temperatures, since the automatic choke system is constructed to open the choke valve early. As a result, driveability of the vehicle decreases at cold engine operation.